1. Field of the Invention
This invention relates to a process for the production of maleic anhydride by the oxidation of nonaromatic hydrocarbons. More particularly, this invention relates to an improved process for the partial oxidation of nonaromatic hydrocarbons in the vapor phase with molecular oxygen or a molecular oxygen-containing gas to maleic anhydride in excellent yields in a heat transfer medium-cooled fixed bed tube-type reaction zone containing a fixed bed of a phosphorus-vanadium mixed oxide oxidation catalyst diluted with an inert solid material in an amount sufficient to form a catalyst-inert solid material composition effective to stabilize the maleic anhydride yield such that the established initial maleic anhydride yield is substantially maintained over an extended period of sustained operations.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these varied needs.
2. Description of the Prior Art
Numerous catalysts containing mixed oxides of phosphorus and vanadium are disclosed in the prior art as being useful for the conversion of various organic feedstocks to maleic anhydride, and further that such catalysts wherein the valence of the vanadium is between about +3.8 and +4.8 are particularly well-suited for the production of maleic anhydride from saturated hydrocarbons having at least four carbon atoms in a straight chain. In many instances, such catalysts also contain added promoter elements which are considered to exist in the catalysts as the oxide. Common organic feedstocks include nonaromatic hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof.
U.S. Pat. No. 4,632,915 discloses catalysts comprising phosphorus, vanadium and oxygen, and a promoter component containing each of iron and lithium which are useful for the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, with molecular oxygen or a molecular oxygen-containing gas in the vapor phase to produce maleic anhydride in excellent yields.
U.S. Pat. No. 4,562,268 relates to a process for the production of maleic anhydride from nonaromatic hydrocarbons in the presence of a phosphorusvanadium mixed oxide oxidation catalyst wherein the catalyst exhibits a single pass weight/weight productivity of at least 70 grams of maleic anhydride per kilogram of catalyst per hour.
U.S. Pat. No. 4,333,853 discloses a phosphorus-vanadium mixed oxide catalyst prepared by reducing vanadium substantially in the pentavalent valence state to a tetravalent valence state in the presence of a phosphorus-containing compound and in the absence of a corrosive reducing agent in an organic liquid medium capable of reducing the vanadium to a valence state less than +5, recovering the resulting vanadium-phosphorus mixed oxide catalyst precursor, drying such precursor, and calcining the precursor to obtain the active catalyst. Such catalysts reportedly are effective in the oxidation of C.sub.4 hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof to produce maleic anhydride with selectivities ranging from 58.7% to 68 1% and yields (mole %) ranging from 51.4% to 59.5%.
U.S. Pat. No. 4,315,864 relates to a process for the production of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a phosphorus-vanadium mixed oxide catalyst. The catalyst is prepared by reducing a pentavalent vanadium-containing compound in an olefinic, oxygenated organic liquid medium to a +4 valence in the absence of a corrosive reducing agent, recovering the resultant catalyst precursor, drying the catalyst precursor, and calcining the precursor to obtain the active catalyst.
U.S. Pat. No. 4,312,787 describes a catalyst which comprises an inert support and a catalytically active mixed oxide material coating of phosphorus and vanadium or of phosphorus, vanadium, and uranium on the outer surface of the support in an amount greater than 50% to about 80% by weight of the combined support and oxide material. Catalysts within the scope of the claims of the patent were reported to produce maleic anhydride from n-butane in yields ranging from 53% to 62.5%, with selectivities ranging from 57.4% to 67.9%.
U.S. Pat. No. 4,294,722 discloses a process for preparing catalysts containing mixed oxides of phosphorus and vanadium. In this process, a pentavalent vanadium-containing compound is reduced (at least in part) to a +4 valent state in an organic liquid medium in which the vanadium compound is at least partially soluble to form a solution or mixture. Any unsolubilized vanadium-containing compound having a particle size greater than 0.1 mm diameter is removed. The resulting solution is mixed with a pentavalent phosphorus-containing compound to form a precipitate which is recovered, dried, and calcined. Such catalysts are reported to be effective in the oxidation of nonbranched C.sub.4 hydrocarbons, such as n-butane, 1- and 2-butenes, 1,3-butadiene, and mixtures thereof, in the presence of molecular oxygen or a molecular oxygen-containing gas in the vapor phase to maleic anhydride with good selectivity.
In U.S. Pat. No. 4,251,390, a zinc-promoted phosphorus-vanadium-oxygen catalyst is disclosed and claimed. The catalyst is prepared by reducing pentavalent vanadium in a substantially anhydrous organic medium to a lower valent state and digesting the reduced vanadium in the presence of a zinc promoter compound. The resulting catalyst is activated by bringing the catalyst to operating temperatures for the oxidation of n-butane to maleic anhydride at a rate of 5.degree. C. to 10.degree. C. per hour in the presence of a butane-in-air mixture.
In U.S. Pat. No. 4,187,235, a process is described for preparing maleic anhydride from n-butane in the presence of a phosphorus-vanadium-oxygen high surface area catalyst, that is, 10 to 100 square meters per gram (BET method). The catalyst is prepared by reducing pentavalent vanadium to a valence between +4.0 and +4.6 with a substantially anhydrous primary or secondary alcohol and contacting the reduced vanadium with phosphoric acid, followed by recovering and calcining the resultant vanadium (IV) phosphate compound.
U.S. Pat. No. 4,018,709 discloses a process for the vapor phase oxidation of normal C.sub.4 hydrocarbons using catalysts containing vanadium, phosphorus, uranium, or tungsten or a mixture of elements from zinc, chromium, uranium, tungsten, cadmium, nickel, boron, and silicon. In a preferred embodiment, the catalyst also contains an alkali metal or an alkaline earth metal, especially lithium, sodium, magnesium, or barium as active components. Typically, such catalysts are prepared in concentrated (37%) hydrochloric acid.
U.S. Pat. No. 4,002,650 discloses a process for the oxidation of n-butane used in a catalyst of the formula: EQU V.sub.0.5-3 P.sub.0.5-3 U.sub.0.1-0.5 O.sub.x
wherein x is a number taken to satisfy the valence requirements of the other elements present. In preferred preparative procedure, a mixture of vanadium pentoxide, concentrated hydrochloric acid, and uranyl acetate is heated under reflux. To this refluxing mixture is added 85% phosphoric acid. The resulting mixture is evaporated at atmospheric pressure and dried at 110.degree. C., ground and screened to a suitable particle size, and activated by heating in an air flow at 482.degree. C. for 16 hours.
In U.S. Pat. No. 3,980,585, a process is disclosed for the preparation of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a catalyst containing vanadium, phosphorus, copper, oxygen, tellurium, or a mixture of tellurium and hafnium or uranium or a catalyst containing vanadium, phosphorus, copper, and at least one element selected from the group of tellurium, zirconium, nickel, cerium, tungsten, palladium, silver, manganese, chromium, zinc, molybdenum, rhenium, samarium, lanthanum, hafnium, tantalum, thorium, cobalt, uranium, and tin, optionally (and preferably) with an element from Groups 1a (alkali metals) or 2a (alkaline earth metals).
U.S. Pat. No. 3,888,866, discloses a process for the oxidation of n-butane by contacting the n-butane at a temperature from about 300.degree. C. to about 600.degree. C. with a phosphorus-vanadium-oxygen catalyst having a phosphorus/vanadium atom ratio of 0.5-2, promoted or modified with chromium, iron, hafnium, zirconium, lanthanum, and cerium, the promoter metal/vanadium atom ratio being between about 0.0025 and about 1. The catalysts are prepared by refluxing a reaction mixture of vanadium oxide, phosphoric acid, a hydrogen halide (usually hydrochloric acid), and a specified promoter metal-containing compound. The resulting catalyst precursors are recovered, dried, formed into structures, and calcined to produce the active catalyst.
U.S. Pat. No. 3,864,280, discloses phosphorusvanadium mixed oxide catalyst having an intrinsic surface area from about 7 to about 50 square meters per gram. The catalysts are prepared by precipitation of a phosphorus-vanadium-oxygen complex from an essentially organic solvent medium in the absence of gross amounts of water. The resulting crystalline precipitate is activated by heating in air followed by a 1.5 mole % butane-in-air mixture, both at elevated temperatures.
U.S. Pat. No. 3,862,146, discloses a process for the oxidation of n-butane to maleic anhydride in the presence of a phosphorus-vanadium-oxygen catalyst complex, promoted or activated with a zinc, bismuth, copper, or lithium activator. The phosphorus/vanadium and activator/vanadium atom ratios are from about 0.5-5 and from about 0.05-0.5, respectively.
U.S. Pat. No. 3,856,824, discloses a process for the production of maleic anhydride by oxidation of saturated aliphatic hydrocarbons in the presence of a catalyst comprising phosphorus, vanadium, iron, oxygen, and added modifier comprising chromium combined with at least one element selected from the group consisting of nickel, boron, silver, cadmium, and barium.
European Patent Application No. 98,039 discloses a process for the preparation of phosphorus-vanadium mixed oxide catalysts, optionally containing an added promoter element selected from the group consisting of Group 1a (alkali metals), Group 2a (alkaline earth metals), titanium, chromium, tungsten, niobium, tantalum, manganese, thorium, uranium, cobalt, molybdenum, iron, zinc, hafnium, zirconium, nickel, copper, arsenic, antimony, tellurium, bismuth, tin, germanium, cadmium, and lanthanides, and mixtures thereof. The catalysts, which exhibit a phosphorus/vanadium atom ratio from about 0.8 to about 1.3 and a promoter/vanadium atom ratio from 0.01 to 0.5, are prepared in organic liquid reaction medium capable of reducing the vanadium to a valent state of approximately +4 to form a nonsolublized catalyst precursor, contacting the nonsolubilized catalyst precursor containing organic liquid with water to form a two-phase system having an upper organic liquid phase and a lower nonsolublized catalyst precursor-containing aqueous phase, drying the catalyst precursor, and calcining. The catalysts so obtained reportedly are useful in the production of maleic anhydride from normal C.sub.4 hydrocarbons.
In general, the use of catalysts described in the prior art for the vapor phase partial oxidation of nonaromatic hydrocarbons to maleic anhydride involves passing a gaseous feed comprising the nonaromatic hydrocarbon and a molecular oxygen-containing gas--n-butane and air, for example--over a fixed bed of the oxidation catalyst in one or more heat transfer medium-cooled fixed bed tube-type reactors at temperatures of about 300.degree. C. to about 600.degree. C. The resulting effluent, containing maleic anhydride, by-product oxygenated hydrocarbons, unreacted nonaromatic hydrocarbon and oxygen, and inert gases is withdrawn from the reaction zone and maleic anhydride is substantially separated therefrom.
In an attempt to improve productivity in nonaromatic hydrocarbon, typically, n-butane, consumption, it has been proposed to employ oxidation feeds containing higher concentrations of the hydrocarbon than are typically employed in conventional operations and/or to recycle reaction zone effluent following separation of a major portion of the maleic anhydride product. For example, U.S. Pat. No. 4,342,699 discloses a vapor phase oxidation process for the production of maleic anhydride from n-butane wherein a butane concentration in the feed stream ranging from about 2 mole % to about 10 mole % is employed. In addition, a suitable catalyst, including phosphorus-vanadium-oxygen catalyst, is employed. Such catalyst is graded in activity along at least a portion of the length of the reaction zone, the lowest activity being at the feed end and the highest activity being at the exit end. Low conversion rates on the order of about 15-70% are attained, thereby necessitating recycling to the reaction zone a major portion of the effluent remaining following recovery of the maleic anhydride.
U.S. Pat. No. 3,899,516 discloses that space time yields and catalyst selectivity to maleic anhydride can be improved through the use of a feed containing n-butane and substantially pure (at least 95%) molecular oxygen in a mole ratio of at least 1:4, that is, at least 20 mole % n-butane and less than 80 mole % molecular oxygen in the feed.
U.S. Pat. No. 3,904,652 discloses the use of feeds containing greater than 1.7 mole percent n-butane, 3-13 mole % molecular oxygen, and 70-95 mole % inert gas, preferably nitrogen, in conjunction with 30% to 70% per pass conversion of n-butane and recycle of reaction effluent after separation of maleic anhydride in order to attain improved selectivity to maleic anhydride and ultimate conversion of n-butane.
In U.S. Pat. No. 4,501,907, a process is described wherein a feed concentration of from about 3 mole % to about 5 mole % n-butane-in-air at space velocities of from about 364 hr.sup.-1 to about 536 hr.sup.-1 are employed.
Although these prior art processes and catalysts generally produce the desired maleic anhydride product, the commercial utility of a catalyst system and a catalytic process is highly dependent upon the cost of the catalyst employed, the conversion of the reactants, the yield of the desired product(s), and the ability of the catalyst to maintain initially achieved yields of the desired product(s) over an extended period of operations without experiencing substantial yield decay, or stated differently, the actual productivity of the catalyst system over an extended period of sustained operations. In many instances, a reduction in the cost of a catalyst system employed in a given process on the order of a few cents per kilogram or pound, a small percent increase in the yield of a desired product, relative to the amount of catalyst required, and/or a decrease in product yield decay over extended periods of sustained operations represent a tremendous commercial economical savings and advantage. Accordingly, research efforts are continually being made to define new or improved catalyst systems and methods and processes of making new and old catalyst systems to reduce the cost and/or upgrade the activity, selectivity, and/or productivity, and extend the useful life of such catalyst systems in such processes. The discovery of the process of the instant invention, therefore, is believed to be a decided advance in the art.